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The Global Burden of Disease (GBD) study examined the impact of 107 major
diseases in a comparative framework including both mortality and disability.
When ‘disability’ was taken into account, the formerly
unrecognised burden of mental disorders became undeniably evident. Mental
disorders ranked as high as cardiovascular and respiratory diseases,
surpassing all cancers combined and even HIV infection. Depressive disorders,
as a single diagnostic category, were the leading cause of disability
worldwide. These results have provided the most powerful scientific support
and advocacy for mental health care, because they highlighted loss of human
productivity due to mental disorders
(Üstün, 1999).

From a scientific point of view, the GBD study is a
‘
meta-synthesis’ of epidemiological information concerning
incidence, prevalence, duration, severity, associated disability, age of
onset, course and treatment rates of major diseases as well as mortality due
to these conditions. These epidemiological indicators are built into an
internally consistent model of the disease, adjusting for disability (i.e.
health levels) throughout the life span. In calculating the burden of disease,
‘
duration’ is a key driver — like the length of mortgage
that drives your monthly payments. We therefore need to study duration in
terms of onset, length and end of an episode with stopwatch precision, and
determine the distribution of each element in the general population.

SCIENTIFIC STUDIES OF DURATION

This issue of the British Journal of Psychiatry publishes an
important article that scientifically explores the duration of depressive
episodes in a general population sample. Spijker et al
(2002, this issue) report that
the median duration of depressive episode in new cases in the general
population was 3 months in half of the cases, and that a fifth of patients
remained depressed after 2 years. This article is important because it brings
further evidence for the natural history of depressive disorders, by studying
new cases of depression in a prospective design with a detailed examination of
life course. Focusing on duration is indeed the right point to start to
desegregate and refine the epidemiological information on one of the most
burdensome diseases in the world.

Two-wave prospective studies are most suitable for calculating the duration
of depressive episodes in the community. Eaton et al
(1997) showed in the 12-year
follow-up of the Baltimore Epidemiologic Catchment Area (ECA) study that the
median duration of episodes was 12 weeks and the mean duration was 27 weeks
for both genders (males 26, female 27 weeks) and that 22% remained chronically
ill. Eaton et al's data were obtained using an additional Life Chart
Interview (LCI) with cognitive anchors, which indicated the psychopathology
across the life span. Although this method yields the best information
available, the 12-year lag may create attrition and discordance between the
two sets of evaluation at year 1 and year 12 (e.g. owing to state-dependent
recall bias). Spijker and colleagues also used the LCI retrospectively on a
prospective sample assessed by the Composite International Diagnostic
Interview (CIDI), but over a 2-year period.

The National Comorbidity Survey (NCS) did not include a longitudinal
follow-up of cases identified in the community, but the diagnostic interview
included questions related to the duration of lifetime episodes of depression
(Kessler et al, 1994).
For the purpose of this analysis, 3% of the participants who had had very long
episodes were excluded. For the remaining 97% of the sample with episodes of
depression, the overall mean duration was 22.6 weeks (further details
available from the author upon request). Kendler et al
(1997), however, found that the
mean time to recovery in 235 women was 13 weeks.

So we now have ‘duration’ data from a handful of studies to
incorporate in the new wave of GBD calculations, which was not the case when
the original GBD study was done: mean durations of 27 weeks
(Eaton et al, 1997),
23 weeks (Kessler et al,
1994), 30 weeks (Spijker
et al, 2002, this issue) and 13 weeks
(Kendler et al, 1997).
Median values were all around 12 weeks, except for Kendler et al's
study (6 weeks). The results provide input to create better models of the
burden of depressive disorders, enabling us to tease out duration, severity
and age at onset. The GBD 1996 results were generally accepted, but
calculations for depressive disorders remained debatable: episode incidence
was modelled for women as 0.29% and for men 0.16%; episode average age at
onset was 37.1 years, with an average episode duration of 6 months
(Murray & Lopez, 1996: pp.
601-606). How plausible are these estimates?

The GBD incidence and prevalence estimates for depressive episodes are low
in comparison with modern psychiatric epidemiological findings: the annual
prevalence of unipolar depression was 12.9% for women in the NCS
(Kessler et al, 1994),
compared with the 1.7% prevalence estimate used in the GBD calculations.
Similarly, the age of onset of 37.1 years is far beyond the common finding of
20-25 years (Burke et al,
1991). Finally, the duration figure of 6 months was based mainly
on clinical textbook knowledge. In the absence of epidemiological data,
Kraepelin's clinical hunch was 6-8 months on average episodes lasting between
8 days and several years (Angst,
1988). Clinical impressions are known to include chronic duration
as a requirement (Rousseau,
2000). Clinical cases are also presumed to be more severe and
longer-lasting than community cases
(Costello, 1990), which is
possibly due to the filtering effect of care-seeking and severity. Indeed, in
the World Health Organization (WHO) Primary Care study we found that among the
primary care attenders with depression 33.5% were still depressed at the end
of a year and first episodes lasted longer
(Üstün & Sartorius,
1995; also further details available from the author upon
request).

These discrepancies between the newly available epidemiological evidence
and the GBD modelling are noteworthy for future research and the next wave of
GBD calculations. The GBD approach logically assumes a linear relation between
incidence, duration and prevalence. The modelling was, however, done with
average duration and single (uniform) severity for all depressive episodes.
The good news is that it is now possible to do more detailed modelling with
the data from these empirical studies, including depression severity, duration
and access to care, as Spijker et al have teased out. It may also be
possible to extend these studies to young adults and children to explore these
parameters in earlier life (e.g. Kaminski
& Garber, 2002), for example ‘college depression’
is usually moderate and of short duration, mainly associated with social
adjustment and grades (Oswalt &
Finkelberg, 1995).

FUTURE RESEARCH RECOMMENDATIONS

In epidemiological modelling, the devil is always in the methodological
detail.

Mental health epidemiology has largely relied on ‘lifetime’
prevalence estimates. Lifetime prevalence is not used in other speciality
areas, and hence is not useful for making comparisons between mental health
and other fields. The GBD study used 1-month prevalence rates, which took into
account the ‘duration criteria’ built into the diagnostic
definition of mental disorders. It will therefore be more useful to focus on
1-month prevalence and decompose information about duration.

The duration of episodes has a log-normal distribution. The best way to
report this variable is through the use of medians, quartiles or logarithmic
values. However, in the GBD project, the arithmetic mean was used for the
estimation of years of life with disability across all the conditions.
So-called dysthymia and ‘double depression’ cases have extreme
duration values (i.e. longer than 2 years) and these may drive the mean
higher. On the other hand, putting all long-term cases into a dysthymia
category or excluding them is not appropriate: irrespective of the diagnostic
validity of dysthymia this manipulation creates a bias, because the burden for
depression is inappropriately lowered owing to the lower disability weight for
dysthymia. A better way of modelling is needed in the next wave of GBD.

It may be important to treat first episodes separately because it has been
consistently shown that first episodes last longer. This may be due to delay
in seeking help, which shows a learning curve enhancement for recurrent
episodes.

The analytical approach to duration needs some methodological innovations
in measurement and in charting the course of episodes (e.g.
Denicoff et al, 2002).
We also need to define subthreshold states, because dichotomisation as case
v. no case is a crude way to analyse the course of illness. To depict
the temporal development of illness we need symptom profiles, functioning data
and adjustment factors as well as response to therapy.

The GBD study indicates that depression is the most disabling and
burdensome condition in the world. Paradoxically, we know little about the
epidemiology of this condition: age at first onset, duration and recurrence
risk. Studying duration is the key to understanding how we can best intervene
during an episode; similarly, age at onset may be a guide for primary
prevention and recurrence risk for secondary prevention. Already we know the
basic associations between gender and duration of depressive episodes. There
is no difference between women and men in terms of duration or recurrence: the
gender difference lies in the first occurrence of depression. Women are more
likely to become depressed earlier in their lifetime.

However, we know little about the predictors of duration. We are ignorant
of even basic information such as whether the poor have longer episodes than
do the rich. By knowing the predictors of duration, we can possibly modify the
recovery process and interventions. Few studies on predictors of duration
exist; yet huge resources are spent on treatment. As noted by Spijker et
al, we do not even have good epidemiological data on how and for how long
depressive episodes persist. It is necessary to have prospective studies on
the distribution and predictors of duration both in community and in clinical
samples.

THE GLOBAL PICTURE

These findings on the duration and epidemiology of depressive disorders
mainly relate to North America and Europe. To understand the global burden of
depressive disorders, taking into account the variations in service provision
and diffent populations' mental health literacy, we still have a long way to
go. The data are scarce or fragmented for many parts of the world. To address
this issue, the WHO has initiated a series of world mental health surveys,
focusing on the epidemiology of mental disorders within a general health
framework (Kessler & Üstün,
2000). These surveys will provide evidence for duration and other
epidemiological indicators, enabling us to build better intervention
strategies to deal with the global burden of depression.

References

Burke, K. C., Burke, J. D., Rae, D. S., et al
(1991) Comparing age at onset of major depression and other
psychiatric disorders by birth cohorts in five US community populations.
Archives of General Psychiatry,
48,
789
-795.

Kessler, R. C., Blazer, D. G., McGonagle, K. A., et al
(1994) The prevalence and distribution of major depression in
a national community sample: the National Comorbidity Survey.
American Journal of Psychiatry,
151,
979
-986.